Aasana (Physical Posture)
Aasana in Raja Yoga
Patanjali, in his Yoga Sutras defines asana as follows:
sthirasukhamaasanam - posture should be steady and comfortable (sutra 2.46).
In Raja Yoga, asana refers to the seated postures suitable for meditation. When one can maintain a steady and comfortable posture for a long time in meditation, then only the mind can be controlled and made free of 'chitta vrittis" or fluctuations of the mind.
Asana in Hatha Yoga
The ability to sit in one posture for long periods of time, as required for meditation, demands a body that is healthy, strong and flexible and a mind that is free of worry and fear. The yogis practicing meditation realized that without preparing the body properly, discomfort and pain soon develop in different parts of the body after a short time in a seated posture. Any physical discomfort naturally hinders the effort of bringing the mind under control. The asanas in Hatha Yoga help develop a strong and flexible body. As per Hatha Yoga Pradeepika (HYP) (a free online translation and commentary is available here) (shloka 1.17) "Prior to everything, asana is spoken of as the first part of hatha yoga. Having done asana one gets steadiness (firmness) of body and mind, freedom from disease and lightness (flexibility) of the limbs". Hatha Yoga is based on the principle that one can enter deeper levels of consciousness by channelizing the movement of certain kinds of subtle forces (prana, kundalini) in the physical body. The goal of practicing the asanas is to make the physical body perfectly healthy for the influx and manipulation of these subtle forces.
Some of the more commonly practiced asanas, including a picture, name of the asana (in Sanskrit, devanagri script and English) and an audio link to the name in Sanskrit, are given here.
Visit my blog for detailed instructions on many of the commonly practiced asanas.
A photo gallery of asanas with Subhash Mittal is available here.